Gain control circuit



July 28, 1936.

E- w FILTER F/L m? R. w. CHESNUT 2,049,195

GAIN CONTROL CIRCUIT Filed Feb. 9, 1935 F /G. 7 9 5 /s 4 GAIN E-W Q CONTROL ji g FILTER GAIN W-E mML/ZER CON TROL F IL TE]? lNVENTOR R. W CHES/VU T ATTORNEY fiatented juiy 28, 1936 PATENT orrice GAIN CONTROL CIRCUIT Roy W. Chesnut, Upper Montclair, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 9, 1935, SerialNo. 5,696

11 Claims.

This invention relates to gain control circuits and particularly to gain control circuits for carrier telephone systems.

One object of the invention is to provide a carrier telephone system with gain control circuits that shall maintain the transmission efiiciency of the system for the various carrier currents substantially constant in a new and improved manner.

Another object of the invention is to provide a carrier telephone system with gain control circuits that shall control the strengths of the carrier currents according to the strength of a pilot frequency current under the control of marginal space discharge devices.

Another object of the invention is to provide a carrier telephone system with gain control circuits that shall employ gas-filled space discharge devices governed according to the strength of a control current for controlling the connection of an amplifier to impedance networks to hold the strengths of the various carrier currents between predetermined limits. 7

A further object of the invention is to provide a carrier telephone system with gain control circuits that shall employ gas-filled space discharge devices governed according to the strength of a control current for operating a condenser potentiometer to connect an amplifier to various impedance networks in the system and maintain the strengths of the various carrier currents between predetermined limits.

In carrier telephone systems, a' plurality of signals are simultaneously transmitted over a common conducting system through the agency of carrier currents of different frequencies. The attenuation of the system changes in accordance with the variations in line conditions. Varia tions in'the line conditions are due to a num- 40 ber of different causes.- The principal cause of variations in line conditions is the change in leakage conditions'of the conductors. The attenuation of open wire conductors is much greater during Wet weather than during dry 45 weather.

It has been customary to hold thestrengths of the carrier currents within predetermined limits by means of attenuation networks operated by relays and switches and governed by a complicated measuring instrument according to the strength of a pilot frequency current. Gain control circuits of this type are expensive to install and to maintain.

According to the present invention, the arrangement of relays and switches under control of a measuring instrument is replaced by gain controlcircuits employing apparatus having few moving parts. The measuring of the strength of the control or pilot current is effected by gasfilled space discharge devices and a simplified cir- 5 cuit under control of gas-filled devices is provided for controlling an amplifier in the line circuit.

The system employed to describe the invention is a carrier current telephone system employing multi-carrier waves for transmitting signals. 10 Carrier waves within one frequency range are employed for transmitting signals in one direction and carrier waves located Within another frequency range are employed for transmitting signals in an opposite direction. A pilot frequency 15 current or control current is transmitted with each range of carrier waves for controlling gain control apparatus to maintain the strengths of the carrier waves between predetermined limits. At a repeater station the east to west line would be controlled by one pilot frequency current and the west to east line would be controlled by the other pilot frequency current.

In the illustrated circuits, attenuation networks are inserted in one side of the line at a repeater station or a terminal station and a gain control amplifier located in the line beyond the attenuation networks has the input circuit thereof selectively connected to the attenuation networks by means of a condenser potentiometer. A line amplifier is preferably inserted in the line beyond the gain control amplifier and beyond the line amplifier is connected a feed-back circuit for. controlling the operation of the condenser potentiometer which connects the input circuit of the gain control amplifier to the attenuation networks.

p The condenser potentiometer is operated by two hysteresis motors to include more or less of the attenuation networks in the input circuit of the 40 gain control amplifier according to the strength of the pilot frequency current on the line beyond the line amplifier. The two hysteresis motors are directly controlled by three gas-filled space discharge devices. The three gas-filled devices are marginal in their operation and are controlled according to the strength of the pilot frequency current. One of the gas-filled devices is broken down and operated when the strength of the pilot frequency current is raised above a predetermined upper limit. Normally, the other two gas-filled space discharge devices are broken down and in operation. When the high level gas-filled device is broken down, a circuit is completed to energize one of the hysteresis motors for so operating the condenser potentiometer as to include more of the attenuation networks in the input circuit of the gain control amplifier.

If the high level gas-filled device is released and the low level gas-filled device is released, a circuit is completed for. operating the other hysteresis motor. Operation of the second hysteresis motor drives the movable element of the condenser potentiometer in a: direction -to reduce or decrease the amount of the attenuationnett? works included in the input circuit of the gain control amplifier. .device and the low level gas-filled device are in released position, and the third gas-filled'device is released, then an alarm is sounded giving the operator at the station warning-that the strengths,

of the carrier waves at the station havebeen. reduced to very low values.

The three gas-filled space discharge devices which control the operation of the two hysteresis l I gain control circuits; and

' Fig; 21s a diagrammatic'viewof gain control circuits constructed in accordance with the .in-"

Vention. l r 7 Referring to Fig.1 of the drawing a carrier telephone vtransmission line comprising conduce tors l and2 is provided with a repeater station 3. The cast towest line of the repeater station 3'comprises a directional'filter t'which selects the carrier waves and the pilot'frequency-current;within the range employed for transmission from the eastto the west; The directional filter 4 is connected to an equalizer 5'by meanslof a transformer 6'; The 1equalizer 5Lis connected to a line amplifier 'lfland ia-second directional filter. 8; Be tween the amplifier landthe equalizer 511s proa vided gain controlapparatus 9 for adjusting'the strengthjof' the carrier waves according to the;

strength: ofthe pilot. frequency current on the. line beyondthe line amplifier 1.

7 The west'to' east line at the repeater station 3, comprises a west to, eastqdirectional' filter l0 which selects the carrier currents and the pilot frequency current located vwithin the range employed' forv transmissionfrom the west to the,

east. The directional filter I0 is connected to an equalizer H by'means of a transformer l2. equalizer His connected to a'line amplifier] 3 and the line amplifier I3 is connected to a second west to east directional filter l4; Between theequalizer llandthe line amplifier l3 provided gaincontrol apparatus I5 which is similar to the gain controlapparatus 9 employed in the east to westfline'of the repeater. The gaincontrol ap'-' paratusli is. controlled in accordance'with the" strength of 'thepilot frequency current on the line beyond'the amplifier I3.

Referring to Fig: 2 of the drawing; the gaincon trol apparatus I5 employed in'the-West tofeast line of therepeater is disclosed'in'detail. Inasmuch as the gain controlapparatus 9 employed in If the high level gas-filled; ing a condenser plate 2|.

the east to west line of the repeater is similar in construction and operation to the gain control" apparatus I5, it is deemed unnecessary to describe the apparatus 9 in detail. Two attenuation networks It and I! which may be of the type disclosed in the above mentioned patent to R. W. Chesnut et al. are provided in theline for con trolling the operation of a gain control space discharge-device -I 8 in; accordance with the operation of:acondenserspotentiometen l9. The condenser potentiometer comprises a movable arm 20 carry- The condenser plate 2| ismovable adjacent to condenser plates 22, 23;

and24: The condenser plate 24 isconnected to thelineonone sideof' the network ll. The con-' denserlplate -23fis connected betweenlthe networks of.C. R. Eckberg, SerialNo.5,71'7, filed February 9,'1935.'. I r I The gain" control amplifier l8 comprises an screen gridelement 28". The grid element 2! of the amplifierj'lil is directly connected to the eondenserplate 2'l of the condenser potentiometer I9; The cathode'26 of the amplifier 58 which is heated in any suitable -manner; is connected to the side of the line opposite to that connected to the plates 22; 23, and 24 of the condenser potentiometer; When the input'circuit of the amplifier- I8 is so connected, it isapparent that movement of'the arm 20 and the--plate-2 l 'of the condenser'potentiometer selectively connects the attenuation networks l6 and IT in the amplifier input circuit to cont'rol-the level of the signals in the amplifieroutput circuit; Plate potential for the amplifier l8-is supplied'by a suitable source of direct current 291 Potential for the screen grid 28f is"providedbyatap-extending from' a r'esistance30; which is connectedacross the source 29; Resistance elements 3| are provided" for biasing the grid 21 of the amplifier 18f The line beyondttheiattenuation network I l is terminated byimeanssofla relativelylarge resistance 32 which anode 25,, a cathode 26, a grid element 2'l'and a matchesnthez'impedanceof network ll; The outl putxcircuit of 'thflfgfiilLCOIltIOl amplifier I8 is con-l nectedsto; theline'amplifier tl3lby means of a transformer:33.1.

Thevarm2fl ofthe condenser.potentiometer l9 is rotated in aforwardiandzin a reverse direction under the control oftwohysteresismotors and 35;, They hysteresis motors 3e and 35 are controlled-by, three gas-filled spacetdischarge devices 36, 31: and 38laccording to the strength of the pilot1frequency current on the line; beyond the mnlifienlt. The hysteresis-motor 35 comprises a horseshoe shapedzfield 'fstructureiil 9 and an iron or steel; rotor- 40 ;which istfixedly' mounted on a shaft-Al; .ArCOil lzris mounted on-the field struc-g 35sarei so= adjustedithafi energization offthe wind-L 7' 70.

ing 42 of the motor '35irotates the shaft: 41 in .one direction= and energi'zation of the winding 144: of

l the I hysteresis motor" 34* rotatesthe shaft 1 4 I: in

an' opposite'direction. The shaft 4 i isconnected to a shaft Mi by suitable reducinggears 21. 'A pinion 48 on" the-shaft 46 meshes with a gear tor-Tile shaft sc -carries, th 29- rth I wheel 59 which is frictionally mounted on as halt denser potentiometer l9. V

A o i i roi d d n' e d f h ha t. for niakinga manualadjustment of the condenser potentiometer lm 5 An armit on the opposite end *of the shaft 50 controls limiting switches' 53 and 54.

devices: 3B, 31', and ears respectively provided-by the resistance. elements/"59; 60, andfil'; 'A trans former '62 which is connected-- to a source of alternating current fifiprovides'heating current. for thecathodes and potential for the anodes. j

i The device '38, which may :be called the high level device, is normallyinop'erative. and is only broken downto transmit: cur-rent. therethroiigh when .thestrength of the pilet frequency cure rent;;rises; above a; predetermined upper- Upon operation of 'thedevice 3.8, a relay 6k is energized for completing a circuit. to operate the hysteresis motor .314; "The circuit completed by the operation of relay 64 may be traced f-romone 'terminal -,ofthe secondary winding :65 .ofthe nnntciit n t r the gainno rolainnl fier- 7 ci n a re vt6-..

transformer 52, coil'fij or the motor 34, limit switch fiend. contact; members oi relay 6.4 to the h r termina of. he secondary winding 6-5. n on-oper tion f he mot ring thearm 2.0 ofthe condens r no nti niot r :19: s; m ved towards the ri ht, aS .-v ..oWod-. n; F oi: the; dra n to in.- oroal th n te nati nne w rhs in luded: the IB. lino en -fi led; .d v-i efit .J r l h ca ed he o ev dev e. i nor n 1 n e down d' hon ntio to no inlete t tor-en e he st ens li. o the lot re uenc c e -oaths elowa ow rent 68" to! he;ot er e m l eLltheoevices re ea es ande ients el a h e yt n n n ensn i le a ti t i .oomii d f n t nst s motor e niot'oi' it as... n o inn tthr n t hrs .i ai o; thes nd nah nna rom n windingllZf-oi motor limitswitchfit contact,

" any:

' winding 65. Thhysteresi's mater ascent-nae arm mot thercondenser potentiometer] 9' towards the left, as'vi'ewe'd inFig. 2 of the drawing, to decrease the attentuation networks included in the input circuit of the gain control amplifier l8.

The gas-filled device 36 is normally in operation to complete a circuit for energizing the relay 68. The device 36 is only released to break the circuit of the relay 68 when the strength of the pilot frequency current on the line is reduced to a very low level. Upon release of the relay 68 a circuit is completed from the secondary winding 6-5 for operating an alarm 61.

In case the arm 20 of the condenser potentiometer I9 is moved to its extreme position towards the right, as viewed in Fig. 2 of the drawing, to increase the attenuation in the line and reaches an extreme position, the arm 52 operates the limit switch 53 to effect operation of an alarm 69 and to stop motor 34 by removing power from wind- Modificat ons i the c c its and n t e r an er ment. and looationior par s may e. mad w thin tri nn ii an i, scope of t iinvv ntion an ill modincat' ns are intendedto. be covered by the appeilde claims.

' what'isjc aimed is: o I v In oomhinat onjn li e transmitting si nal currents, anda control cur ent, i p a ce means on. saidiine ror contro l n th strength ofihe on rent 'on. the line ineasnrinemeans. omp e ins; two" e s-fi led s at iiistliiirgfe d s oe spectivelyjonorated by asingle backward acting control circuit at upper and lower limitshi n reng hoi the oont olo rr nt 'a d niot means operat d} [forw rd and na reverse direction by sa d meaning means Ito. govern aidiimpedmeans nndno d he stren t or the line currents between, predetermin d upper a d owe lini ts.

.2; Int nation a. linelt a smittina ni nal cu rents an, a control.) il AR att nu t on netw rks. on said lineraisnaoe discharge am hiiei; ni he beyond sai networks a. condenser poten iometer o o' re in the inpnticircuit or Sa d ninliiier sele t en tosa d e w rkstocom r l. the str n th the cmon s on o'line eyond. 8 iiiamn ifi'er, measur n means domnris ins. n -.,,1.l d. space di harge. devic perat d acco din t the stre gt f tho oont ol' cu ent and. mom? mo ns ontrol d by said measur mea s. o g verning said condenser r otentb ometer t hold; the. strength of, theiine Currents eyo d d. amplifier between predetermined imi s.

'3. In combination,' a line transmitting signal aves and a, cont l cu r nt attenuati n-I n t works on Said, lin airspace} dischar e amplifier o the line bey n Said. netw rks}, a condenser potentiometer for connecting the input'circuit of the amplifier to the line andi to selectively" con nec sa d netwo ks in. circuit. w th sai am'r ifier,

nd. means for opera ing, said condenser potenti n eteraccording the, strength, ofv theoontfrol ur ent. to. controlthe stren th of the si nal] Wnv sbey nd saidamnl fierr v 4, Ihcgmbination, a, line transmitting modue. lated carrier waves} and, a pilot. frequency cur... renthatt nna ion netwo kso aid li pace. d so 'ee. amplifier ha in u pu rcuit. on th l ne b y ndv saidamnfiii tr to thelin at'variousro nts; wit resp ct. to, sa'id'gnetwork an; ror c trolling said condenser potentiometer according to the strength of the pilot current to govern the strength of the modulated carrier waves beyond said amplifier.

5. In combination, a line transmitting modulated carrier waves and a pilot frequency current, attenuation networks on said line, a condenser potentiometer connected to the line adjacent to said networks, a space discharge amplifier having an output circuit in the line beyond said networks and having an input circuit connected to the line by said condenser, means comprising marginal space discharge devices for controlling said condenser potentiometer to govern the operation of said amplifier, and means for controlling said marginal devices according to the strength of the pilot current on the line beyond said amplifier.

H a aidjnetworks, a1 condensate-g tent'iomet'er for connecting thejinput circuit at to control the strength oftheicurren 6. In combinations line transmittingjsi'gnal waves and a controlcurrent', impedance means comprising a space discharge, amplifier for con trolling the strength of thejcurrents on the line, motor means operated in ajforward and in'a reverse direction to control, said impedance means, means comprising two marginal space discharge devices for'operating said mot line beyond said amplifier,saidi'devicesl p tively breaking down at uppefaindilowei limits in the strength of said control current, and means operatednpon "breakdown of one, ofsaid'neviees forcontrolling said motor means tmqwemne strength' or the currents, on the line and operated upon release, or the other device. or controlling said motor means to increase the strength "of ithe currents on the line;v I

7. 'Inzcombination; ajline transmitting signal waves and a control current; impedance; means for controlling the strength'of the currents on said line, motor meansoperatedin a forward and in a'reverse direction for'controllingv said impedance means, means comprising two gas-filled space dischargedevicesfor controlling said motor meansjto govern the strength of the currentson said line," saiddevices respectively loreakingdow'ri at upper and lower limits inthe strength of said 7 control current, and means operated upon breakdown of the device breaking down at the 7 upper limitin 'theflstrengthof the control cm- I discharge devices according "to the st rent for'operating said motojr means-in one di said line, motor meansfoperated in a 'forwar'd and in a reverse direction for controlling said impedance means to govern thestrengthj of the currentson the linegmeans comprising two g'a'sfilled space discharge devicesgforoperating said motor means in a forward and ma reverse dir'c tion, one of said devices, operatin'gfto' efie'c't' operation of the motor means incne" direction and the other or said devicesgceasing' toj operatejtof effect operation of the'motor means in a reverse" direction, and'means for controlling said space r'engthot the control current; j V

' QQIncombinatiQn, a lineftransmitting signal current and a'control current, attenuation net-j works on said line, a space discharge amplifier on theline beyondsaid networks, control means .for; connecting the input circuitlot said amplifierselectively to said networks to. control the strength of the currents on the linebeyondsaid 5 amplifier, means comprising two gas-filled space discharge devices selectively operated according to the strength of thewcontrol current'on the line beyond said amplifiensaid devices respectively breaking down at upper and lower, limits in the strength of saidcontrol current, motor means for operatingsaid control means and means for op'erating said motormeansupon release Of 16 of said devices and upon operation of the other device-for governing :said control means to hold the strength of the line currents beyond ,said amplifier fbetween predetermined limits. i H

10. ,In combinatioma line transmitting modulated carrier waves and a control current, attenuation networks on said line, a; space discharge amplifier on the line-beyond said networks,- a condenser potentiometer for connecting the input circuit of said amplifier tofisaidr networks to control the strength of therline currents beyond 25 said? amplifier, motor'means'forcontrolling said 7 potentiometer, means comprising gas-filled spaoe discharge devices for controlling said motor means, and means for selecting the control current'from the output of said amplifier. to control 30 the gas-filled space discharge devices and to hold the strength of the line current beyond th amplifier-between predeterminedlimits.

111 In combinatioma line transmitting signal 7 waves and'a control current; impedance means 35 on saidline for controlling the strength of the s currents on the line, three gas-filled space discharge devices-operating'as marginal devices, means for operaating said devices according to the strength or the control'current on the line, 4

control currents fallsbelow the first lower limit,

and for operating said'alarm when the strength. of, theicontrol currentfallshelow the second lower limit.

' 'LROY'W. cmrsno'r. 

